Permanent magnet electrical machines whether used as motors or generators are relied upon in safety critical situations. Thus, the electrical machine must continue to operate despite shorted turns or coils in the armature winding of the machine. Typically, these permanent magnet electrical machines have modular windings in which there is physical and electromagnetic separation of coils wound around alternate armature teeth. It is this separation of the armature coils which leads to the term modular winding. For truly fault tolerant electrical machines, there is a general requirement for an armature reactance of around 1.0 per unit and so this requires a suitable modular power converter, e.g. H-bridge converter unit for each phase.
With a fault tolerant permanent magnet machine, it will be understood that the machine can continue to operate or tolerate an open circuit or short circuit of one phase whilst the remaining phases can continue at or near normal rated power. In such circumstances, the electrical machine can continue to function until it is possible to repair or replace it.
In addition to short circuit failures at electrical machine terminals, it is also desirable that permanent magnet machines with modular windings are tolerant to short circuits in individual turns. However, in order to be so tolerant, it has been suggested in the prior art that the turn-to-turn fault is recognised immediately using an appropriate intelligent monitoring system such that when recognised, the entire affected phase is short circuited. If recognition and short circuiting is not so performed, then the electrical current in the shorted turns will be excessively high and could jeopardise the entire winding immediately or compromise its reliability for future performance. In short, the shorted turns or the winding are isolated to protect the whole machine's ongoing operational integrity. Nevertheless, where there are few turns per winding coil, it will be appreciated that it may be difficult to use normal approaches to shorted turn protection in a fault tolerant electrical machine.